One of the major unsolved problems in the life sciences is the regulation of gene activity in eukaryotes. The hormonal control of differentiation of imaginal discs of Drosophila has many features which make it a model system for the study of gene regulation. Besides the obvious possibility of applying the advanced technology of Drosophila genetics, biochemical studies are facilitated by the fact that apparently normal differentiation occurs in defined medium upon exposure to the steroid hormone 20-hydroxecdysone. Present results suggest that the mechanism of action of 20-hydroxyecdysone is basically similar to that of vertebrate steroids and that a more detailed investigation in Drosophila is technically feasible. Ecdysteroid receptors have been identified in imaginal discs. The parameters of the observed binding correlate well with morphogenetic and synthetic responses to hormone. The objectives of the proposed research are to investigate in some detail the binding to nuclear and cytoplasmic components of imaginal discs and to identify the components responsible for the morphogenetic and synthetic responses. In addition techniques will be developed to apply similar analysis to the mechanism of ecdysteroid regulation of gene activity in mass isolated Drosophila salivary glands. The availability of a new labeled hormone analog should make possible an autoradiographic investigation of the distribution of hormone binding sites on salivary gland polytene chromosomes. Preparations for genetic analysis will continue with the selection of appropriate temperature sensitive mutants. Techniques designed specifically to recover ecdysteroid binding mutants will be employed.